Optical coupler including a faraday rotator layer and at least one grating coupler

ABSTRACT

An optical coupling device can couple incident light from a fiber into waveguides, but can reduce the coupling of return light from the waveguides into the fiber. A Faraday rotator layer can rotate by forty-five degrees, with a first handedness, respective planes of polarization of incident beams, and can rotate by forty-five degrees, with a second handedness opposite the first handedness, respective planes of polarization of return beams. A redirection layer can include at least one grating coupler that can redirect an incident beam of one polarization so that the redirected path extends within the redirection layer toward a first waveguide, and can redirect an incident beam of an opposite polarization so that the redirected path extends within the redirection layer toward a second waveguide. An optional birefringent layer can spatially separate incident beam having different polarizations, so that two single-polarization grating couplers can be used.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.16/727,525, filed Dec. 26, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/685,114, filed Aug. 24, 2017, all of which areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present disclosure relates to a device for coupling light between anoptical fiber and waveguides.

BACKGROUND

An on-chip light source can deliver light to a photonic integratedcircuit. An optical fiber can deliver the light from the photonicintegrated circuit. It is desirable to reduce or eliminate lightdirected back from the optical fiber to the on-chip light source, whichcan cause noise or other instability in the light from the on-chip lightsource.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description of example embodiments of theinvention, reference is made to the accompanying drawings which form apart hereof, and which is shown by way of illustration only, specificembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

FIG. 1 shows an example of a device that can couple incident light froma fiber into waveguides, and can reduce the coupling of return lightfrom the waveguides into the fiber, in accordance with some embodiments.

FIG. 2 shows an example of incident light propagating through the deviceof FIG. 1, in accordance with some embodiments.

FIG. 3 shows an example of return light propagating through the deviceof FIG. 1, in accordance with some embodiments.

FIG. 4 shows an example of a dual-polarization grating coupler, inaccordance with some embodiments.

FIG. 5 shows another example of a dual-polarization grating coupler, inaccordance with some embodiments.

FIG. 6 shows an example of another device that can couple incident lightfrom a fiber into waveguides, and can reduce the coupling of returnlight from the waveguides into the fiber, in accordance with someembodiments.

FIG. 7 shows an example of incident light propagating through the deviceof FIG. 6, in accordance with some embodiments.

FIG. 8 shows an example of return light propagating through the deviceof FIG. 6, in accordance with some embodiments.

FIG. 9 shows an example of two single-polarization grating couplers, inaccordance with some embodiments.

FIG. 10 shows another example of two single-polarization gratingcouplers, in accordance with some embodiments.

FIG. 11 shows an example of another device that can couple incidentlight from a fiber into waveguides, and can reduce the coupling ofreturn light from the waveguides into the fiber, in accordance with someembodiments.

FIG. 12 shows an example of incident light propagating through thedevice of FIG. 11, in accordance with some embodiments.

FIG. 13 shows an example of return light propagating through the deviceof FIG. 11, in accordance with some embodiments.

FIG. 14 shows an example of incident light propagating through thedevice of FIG. 11 when the device of FIG. 11 is configured as atransmission element, in accordance with some embodiments.

FIG. 15 shows an example of return light propagating through the deviceof FIG. 11 when the device of FIG. 11 is configured as a transmissionelement, in accordance with some embodiments.

FIG. 16 shows an example of a method for coupling incident light from afiber into waveguides, and reducing the coupling of return light fromthe waveguides into the fiber, in accordance with some embodiments.

Corresponding reference characters indicate corresponding partsthroughout the several views. Elements in the drawings are notnecessarily drawn to scale. The configurations shown in the drawings aremerely examples, and should not be construed as limiting the scope ofthe invention in any manner.

DETAILED DESCRIPTION

Some optical devices can include an on-chip light source, such as alaser. A photonic integrated circuit can include one or more waveguidesthat can direct light from the on-chip light source to other portions ofthe chip, as needed. To direct light away from the chip, a couplingdevice can couple light from one or more of the waveguides to an opticalfiber. Reflections from one or both ends of the fiber can feed back intothe waveguides, and can create spurious lasing effects or additionalnoise in the light output from the on-chip light source.

The optical coupling device discussed in detail below can reduce oreliminate such unwanted feedback, by reducing or eliminating coupling oflight of the same polarization state back into the waveguide (e.g., froma waveguide, to the fiber, and back to the waveguide). For example, anon-chip light source can produce light. The light can pass from a firstwaveguide, to the grating coupler or couplers, to the fiber. If thelight returns from the fiber without any change in polarization state,as would be the case from a reflection from an end of the fiber, thenthe light returns to the grating coupler or couplers, then passes to asecond waveguide, which is different from the first waveguide. Thesecond waveguide directs the light from the fiber away from the on-chiplight source, which can reduce or eliminate the spurious lasing effectsor additional noise in the light output from the on-chip light source.The device can be formed as a layered structure, which can takeadvantage of wafer-level processes to reduce a cost of the device andimprove the device-to-device uniformity of the device.

Although the optical coupling device may be used to reduce feedback foran on-chip light source, with an optical path extending from a waveguideto a fiber, and back to the waveguide, it is simpler to discuss theoptical path and the optical components starting with the fiber, andextending toward the waveguides. The discussion in the remainder of thisdocument uses such a convention, where light propagating from a fiber toa waveguide is “incident” light, and light propagating from thewaveguide to the fiber is “return” light. Such a naming conventionassumes that the light is delivered via the fiber, but it will beunderstood that the optical devices discussed below can also be usedwith an on-chip light source coupled to one of the waveguides.

In a first configuration, shown in FIGS. 1-3, the device can use aforty-five-degree Faraday rotator to manipulate a polarization state ofbeams propagating through the device, so that light with a particularpolarization orientation can return having an orthogonal polarizationorientation. Because lasing effects can be sensitive to polarizationstate, this configuration can help reduce spurious lasing effects. Theconfiguration of FIGS. 1-3 uses a dual-polarization grating coupler.

In a second configuration, shown in FIGS. 6-8, the device improves overthe first configuration by adding a birefringent layer to spatiallyseparate the two polarization states, and uses two single-polarizationgrating couplers instead of a single dual-polarization grating coupler.Such single-polarization grating couplers can have a higher couplingefficiency than a comparable dual-polarization grating coupler, so thatthe overall coupling efficiency of the device can be improved. In thefirst and second configurations, undesirable return light returns withan orthogonal polarization state.

In a third configuration, shown in FIGS. 11-13, the device improves overthe first and second configurations by reversing the order of theFaraday rotator layer and the birefringent layer. Doing so can change aphysical optical path traversed by return light in the birefringentlayer, and can allow the device to direct the return light to adifferent physical location. Directing the return light to a differentphysical location can dramatically reduce the coupling, and can providean additional degree of optical isolation for the coupling device, inaddition to rotating the plane of polarization.

It is instructive to clarify the terms “light” and “path”, as used inthis document. Light (e.g., such as first light, second light, and soforth) is intended to represent a beam of light, or a collection oflight rays. A beam can have a cross-sectional size, which can bereferred to as a footprint. A beam can be collimated, so that thefootprint remains at a constant size as the beam propagates. A beam canbe converging, so that the footprint shrinks as the beam propagates. Abeam can be diverging, so that the footprint grows as the beampropagates. A beam can change in cross-sectional size, divergence, anddirection as it propagates throughout an optical system. In thisdocument, a beam can emerge from an optical fiber and couple into awaveguide, or can emerge from a waveguide and couple into an opticalfiber. A path is intended to represent a central axis of a beam as itpropagates. The beams can propagate along the paths, in both an incidentdirection (fiber-to-waveguide) and a return direction(waveguide-to-fiber).

Throughout this document, there are four lights (e.g., four beams) thatare used repeatedly among the various system configurations. Forconvenience, these four lights are designated as first, second, third,and fourth, although there is no significance to the numbering order. Inthe figures, the first, second, third, and fourth lights are labeledwith encircled numerals one, two, three, and four, with black numeralson a white background.

There are also two paths that are used repeatedly among the varioussystem configurations. For convenience, these two paths are designatedas first and second, although there is no significance to the numberingorder. In the figures, the first and second paths are labeled withencircled numerals one and two, with white numerals on a blackbackground. The first and second optical paths can overlap in oneportion of the optical coupling device, and can diverge in anotherportion of the optical coupling device.

The following paragraph defines the four lights and two paths usedthroughout the document. The paragraph can also provide a summary of allthree system configurations in generality. The details of eachconfiguration are provided in the discussion below.

In an example, a device can include a Faraday rotator layer. The Faradayrotator layer can receive first light having a first polarizationorientation. The Faraday rotator layer can receive second light having asecond polarization orientation orthogonal to the first polarizationorientation. The Faraday rotator layer can propagate the first lightthrough the Faraday rotator layer to form third light. The third lightcan have a third polarization orientation angled by forty-five degreeswith respect to the first polarization orientation. The Faraday rotatorlayer can propagate the second light through the Faraday rotator layerto form fourth light. The fourth light can have a fourth polarizationorientation angled by forty-five degrees with respect to the secondpolarization orientation. The device can further include a planarredirection layer. The redirection layer can include at least onegrating coupler. The redirection layer can redirect the third light topropagate along a first redirected path within the redirection layer andredirect the fourth light to propagate along a second redirected pathwithin the redirection layer. The second redirected path can be angledwith respect to the first redirected path. A first optical path canextend from a fiber, through the device, to a first waveguide. A secondoptical path can extend from the fiber, through the device, to a secondwaveguide.

The preceding paragraph is merely a summary of the subject matter thatfollows, and should be not be construed as limiting in any way. Thefollowing discussion provides additional details regarding eachconfiguration.

FIG. 1 shows an example of a device 100 that can couple incident lightfrom a fiber into waveguides, but can reduce the coupling of returnlight from the waveguides into the fiber, in accordance with someembodiments. FIG. 2 shows an example of incident light propagatingthrough the device 100 of FIG. 1, in accordance with some embodiments.FIG. 3 shows an example of return light propagating through the device100 of FIG. 1, in accordance with some embodiments. FIGS. 1-3 arediscussed in tandem below.

A Faraday rotator layer 112 can receive first light having a firstpolarization orientation. The Faraday rotator layer 112 can receivesecond light having a second polarization orientation orthogonal to thefirst polarization orientation. The Faraday rotator layer 112 canpropagate the first light through the Faraday rotator layer to formthird light. The third light can have a third polarization orientationangled by forty-five degrees with respect to the first polarizationorientation. The Faraday rotator layer 112 can propagate the secondlight through the Faraday rotator layer 112 to form fourth light. Thefourth light can have a fourth polarization orientation angled byforty-five degrees with respect to the second polarization orientation.

A planar redirection layer 116 can include at least one grating coupler118. The redirection layer 116 can redirect the third light to propagatealong a first redirected path 120 within the redirection layer 116 andredirect the fourth light to propagate along a second redirected path122 within the redirection layer 116. The second redirected path 116 canbe angled with respect to the first redirected path 120. In someexamples, the first redirected path can be orthogonal to the secondredirected path. In some examples, on the first and second redirectedpaths 120, 122, the redirected third light and the redirected fourthlight can have respective polarization orientations that are parallel toa plane of the redirection layer 116.

A first optical path can extend from a fiber 102, through the device100, to a first waveguide 106. A second optical path can extend from thefiber 102, through the device 100, to a second waveguide 108.

The components of the device 100 are discussed in further detail below.

An optical fiber 102 can deliver light 104 from an off-chip lightsource, such as one or more laser diodes and/or one or more lightemitting diodes. In some examples, the light 104 from the fiber iscontinuous-wave. In other examples, the light 104 from the fiber caninclude a data stream modulated onto the light. The optical fiber 102can be a single-mode fiber or a multi-mode fiber, with any suitable coreand cladding configuration. Whereas the device 100 can be a layeredstructure, formed from a stack of generally planar layers, the opticalfiber 102 can deliver the light 104 to the device 100 through a top (orbottom) surface of the layers, so that the delivered light can propagatethrough the layers. In some examples, the optical fiber 102 can bepositioned to direct the light 104 into the device 100 along alongitudinal axis, the longitudinal axis being orthogonal to a plane ofone or more of the layers. In other examples, the optical fiber 102 candirect the light 104 into the device 100 at an angle with respect to thelongitudinal axis. In some examples, the optical fiber 102 is not partof the device 100. In other examples, the optical fiber 102 can beconsidered to be part of the device 100.

In some examples, an optional focusing layer 110 can impart aconvergence onto the first light such that the redirected third light isfocused onto a longitudinal end of the first waveguide 106. The optionalfocusing layer 110 can impart a convergence onto the second light suchthat the redirected fourth light is focused onto a longitudinal end ofthe second waveguide 108. The convergence can impart a mode shape thatefficiently couples the energy into waveguide modes. In some examples,the focusing layer 110 can include a first surface that includes one ormore curved features, and a second surface contacting the first surfaceof a Faraday rotator layer 112, or of another suitable layer downstreamfrom the focusing layer 110. In some examples, the focusing layer 110can include a lens. The lens can include optically isotropic materials,such as silica or air. The lens can include one or more surfaces. Eachsurface can have zero curvature (e.g., flat) or a finite curvature. Insome examples, the lens can be attached directly to a photonicintegrated circuit, either on a substrate surface or on a surface thatincludes waveguides. In some examples, a lens surface can be external tothe photonic integrated circuit, separated by an air gap. In someexamples, birefringent material can be attached directly to the photonicintegrated circuit. In some examples, the lens and the birefringentmaterial can be attached to one another but separated from the photonicintegrated circuit by an air gap. In some examples, the lens can beattached to the birefringent material, which is attached to the photonicintegrated circuit.

In the geometry of the configuration of FIGS. 1-3, the first light andthe second light can be coincident along a single optical path. In theconfiguration of FIGS. 1-3, the redirection layer 116 can include adual-polarization grating coupler 118. The dual-polarization gratingcoupler 118 can redirect the third light to propagate along the firstredirected path 120 within the redirection layer 116 and redirect thefourth light to propagate along the second redirected path 122 withinthe redirection layer 116.

The Faraday rotator layer 112 can be positioned in the first and secondpaths adjacent the optical fiber 102, or if the focusing layer 110 ispresent, adjacent the focusing layer 110. The Faraday rotator layer 112can rotate by forty-five degrees, with a first handedness, respectiveplanes of polarization of the incident beams (FIG. 2). The Faradayrotator layer 112 can rotate by forty-five degrees, with a secondhandedness opposite the first handedness, respective planes ofpolarization of the return beams (FIG. 3). In the configuration of FIGS.2 and 3, the Faraday rotator layer 112 imparts a forty-five degreerotation to the incident beams using a so-called left-hand screw rule,and imparts a forty-five degree rotation to the return beams using aso-called right-hand screw rule. In other configurations, the left-handand right-hand screw rules can be swapped. In all of theseconfigurations, the net effect of the Faraday rotator is that if lightemerges from the optical fiber 102 with one polarization, it can reflectfrom a waveguide and return to the optical fiber 102 with an orthogonalpolarization.

An optional spacer layer 114 can be positioned in the first and secondpaths adjacent the Faraday rotator layer 112. In some examples, thespacer layer 114 can be formed as a substrate, upon which a photonicintegrated circuit can be formed. In some examples, the spacer layer 114can be formed from an isotropic (e.g., non-birefringent) material. Insome examples, the spacer layer 114 can have a first surface thatcontacts a second surface of the Faraday rotator layer 112.

The planar redirection layer 116 can be positioned in the first andsecond paths adjacent the Faraday rotator layer 112, or if the spacerlayer 114 is present, adjacent the spacer layer 114. The redirectionlayer 116 can include at least one grating coupler. In some examples,the at least one grating coupler can be disposed on a surface of theredirection layer 116, such as on a top surface or a bottom surface ofthe redirection layer 116. In some examples, the at least one gratingcoupler can be disposed in a volume of the redirection layer 116,between the top and bottom surfaces of the redirection layer 116.

In the configuration of FIGS. 1-3, the redirection layer 116 includes adual-polarization grating coupler 118. (In other configurations, such asthose shown in FIGS. 6-8 and FIGS. 11-13 and discussed below, theredirection layer 116 can alternatively include two single-polarizationgrating couplers.) The dual-polarization grating coupler 118 can beconfigured such that the first and second paths coincide between theoptical fiber 102 and the dual-polarization grating coupler 118 (e.g.,from the optical fiber 102, through the optional focusing layer 110,through the Faraday rotator layer 112, through the optional spacer layer114, to the dual-polarization grating coupler 118 at the redirectionlayer 116). The dual-polarization grating coupler 118 can redirect thethird light (FIG. 2) to extend within the redirection layer 116 towardthe first waveguide 106 on a first redirected path 120 (e.g., from thedual-polarization grating coupler 118 to the first waveguide 106). Thedual-polarization grating coupler 118 can redirect the fourth light(FIG. 2) to extend within the redirection layer 116 toward the secondwaveguide 108 on a second redirected path 122 (e.g., from thedual-polarization grating coupler 118 to the second waveguide 108). Thesecond redirected path 122 can be angled with respect to the firstredirected path 120. In some examples, the first redirected path 120 canbe orthogonal to the second redirected path 122. Other angles can alsobe used. In some examples, on the first and second redirected paths 120,122, the redirected third light can have a polarization orientationparallel to a polarization orientation of the redirect fourth light. Insome examples, on the first and second redirected paths 120, 122, theredirected third light and the redirected fourth light can haverespective polarization orientations that are parallel to a plane of theredirection layer 116. In other examples, on the first and secondredirected paths 120, 122, the redirected third light and the redirectedfourth light can have respective polarization orientations that areorthogonal to a plane of the redirection layer 116.

FIG. 4 shows an example of a dual-polarization grating coupler 118Asuitable for use in the device 100 of FIGS. 1-3, in accordance with someembodiments.

The dual-polarization grating coupler 118A can includes a region 402 inwhich a refractive index varies periodically in two directions, a firstof the two directions being orthogonal to the first redirected path 120,a second of the two directions being orthogonal to the second redirectedpath 122. The region 402 can be sized larger than respective footprints404 of the first and second beams.

FIG. 5 shows another example of a dual-polarization grating coupler 118suitable for use in the device 100 of FIGS. 1-3, in accordance with someembodiments.

Compared with the dual-polarization grating coupler 118A of FIG. 4, thedual-polarization grating coupler 118B of FIG. 5 has the additionalfeature that the periodic variations in refractive index can be curvedin shape. The curvature can focus the first and second beams ontolongitudinal ends of the first and second waveguides 106, 108,respectively. The focusing effects of such curvature can augment orreplace the focusing function of the optional focusing layer 110.

A potential improvement over the configuration of FIGS. 1-3 would be touse two single-polarization grating couplers, rather than a singledual-polarization grating coupler 118. A single-polarization gratingcoupler can be designed to have a high coupling efficiency for just onepolarization state, which is generally easier than designing to havehigh coupling efficiencies simultaneously for two orthogonalpolarization states. In order to use two single-polarization gratingcouplers, the device can add a birefringent layer that imparts a spatialseparation between beams having two orthogonal polarization states. Sucha configuration that uses a birefringent layer and twosingle-polarization grating couplers is shown in FIGS. 6-8.

FIG. 6 shows an example of another device 600 that can couple incidentlight from a fiber into waveguides, but can reduce the coupling ofreturn light from the waveguides into the fiber, in accordance with someembodiments. FIG. 7 shows an example of incident light propagatingthrough the device 600 of FIG. 6, in accordance with some embodiments.FIG. 8 shows an example of return light propagating through the device600 of FIG. 6, in accordance with some embodiments. FIGS. 6-8 arediscussed in tandem below.

Compared with the device 100 of FIGS. 1-3, the device 600 of FIGS. 6-8can further include a birefringent layer 602 positioned between theFaraday rotator layer 112 and the redirection layer 116. Thebirefringent layer 602 can impart a spatial separation between the thirdlight and the fourth light, such that the third light and the fourthlight coincide upon exiting the Faraday rotator layer 112, the thirdlight is angled with respect to the fourth light within the birefringentlayer 602, and the third light and the fourth light are parallel andspatially separated between the birefringent layer 602 and theredirection layer 116.

A birefringent layer 602, positioned in the first and second paths, canimpart a spatial separation between the first and second paths, suchthat the first and second paths coincide between the optical fiber 102and the birefringent layer 602 (e.g., from the optical fiber 102,through the optional focusing layer 110, and through the Faraday rotatorlayer 112, to a first surface of the birefringent layer 602 that facesthe Faraday rotator layer 112). The first path can be angled withrespect to the second path within the birefringent layer 602 (e.g. fromthe first surface of the birefringent layer 602, to a second surface ofthe birefringent layer 602 that opposes the first surface). The firstand second paths can be parallel and spatially separated between thebirefringent layer 602 and the at least one grating coupler (e.g., fromthe second surface of the birefringent layer 602, through the optionalspacer layer 114, to the grating couplers at the redirection layer 116).

For an incident light beam that is orthogonal to the first surface ofthe birefringent layer 602, the first path can correspond to an ordinaryray, which propagates undeflected in the birefringent layer 602, whilethe second path can correspond to an extraordinary ray, which angularlydeflects toward or away from the optic axis (OA) in the birefringentlayer 602. For positive uniaxial birefringent materials, such as rutile,the extraordinary ray angularly deflects toward the optic axis (OA). Fornegative uniaxial birefringent materials, such as calcite and lithiumniobate, the extraordinary ray angularly deflects away from the opticaxis (OA). In FIG. 6 and subsequent figures, the birefringent layer 602is formed from one or more positive uniaxial materials. It will beunderstood that negative uniaxial materials can alternatively be usedfor the birefringent layer 602. For both positive uniaxial materials andnegative uniaxial materials, the angular deflection lies in the planeformed by an optic axis (OA; FIG. 7) of the birefringent layer 602 and alongitudinal axis (LA; FIG. 7) that is orthogonal to a plane of theredirection layer 116. In some examples, the birefringent layer 602 caninclude calcite, which has a relatively large birefringence. In someexamples, the birefringent layer 602 can include calcite or lithiumniobate, which are typically available in a wafer format from whichpieces can be singulated to form the layered structure. In someexamples, the first birefringent layer 602 can include rutile, which isalso typically available in wafers, and can optionally be depositeddirectly on a wafer of a photonic integrated circuit.

In addition, the redirection layer 116 can include twosingle-polarization grating couplers 604, 606 having a spatialseparation that matches the spatial separation between the third lightand the fourth light. A first 604 of the two single-polarization gratingcouplers can redirect the third light to propagate along the firstredirected path 120. A second 606 of the two single-polarization gratingcouplers can redirect the fourth light to propagate along the secondredirected path 122.

In some examples, the first redirected path 120 can lie within a planeformed by an optic axis (OA) of the birefringent layer and alongitudinal axis (LA) that is orthogonal to a plane of the redirectionlayer 116. In some examples, the second redirected path 122 can beorthogonal to the plane formed by the optic axis (OA) and thelongitudinal axis (LA). In some examples, on the first and secondredirected paths 120, 122, the redirected third light and the redirectedfourth light can have respective polarization orientations that areparallel to a plane of the redirection layer 116.

FIG. 7 shows the paths and polarization states of incident light (e.g.,light propagating from fiber-to-waveguides). FIG. 8 shows the paths andpolarizations states of return light (e.g., light propagating fromwaveguides-to-fiber). The Faraday rotator 112 can ensure that light froma fiber can propagating to a waveguide and return to the fiber with anorthogonal polarization state. The birefringent layer 602 can spatiallyseparate two orthogonal polarization states. The spatially separatedpolarization states can allow the use of two single-polarization gratingcouplers 604, 606, which can have an improved coupling efficiency over acomparable single-polarization grating coupler.

A planar redirection layer 116, positioned in the first and secondpaths, can include two single-polarization grating couplers 604, 606having a spatial separation that matches the spatial separation betweenthe first and second paths. A first 604 of the two single-polarizationgrating couplers can redirect the first and third beams so that thefirst path extends within the redirection layer 116 toward the firstwaveguide 106 on a first redirected path 120 (e.g., from the firstsingle-polarization grating coupler toward the first waveguide 106within the redirection layer 116). A second 606 of the twosingle-polarization grating couplers can redirect the second and fourthbeams so that the second path extends within the redirection layer 116toward the second waveguide 108 on a second redirected path 122 (e.g.,from the second single-polarization grating coupler toward the secondwaveguide 108 within the redirection layer 116). The second redirectedpath 122 can be angled with respect to the first redirected path 120. Insome examples, the second redirected path 122 is orthogonal to the firstredirected path 120.

In some examples, on the first and second redirected paths 120, 122, thefirst and second beams have respective polarization orientations thatare parallel to a plane of the redirection layer 116. In other examples,on the first and second redirected paths 120, 122, the first and secondbeams can have respective polarization orientations that areperpendicular to a plane of the redirection layer 116, or angled withanother suitable orientation.

In the configuration of FIGS. 6-8, the optical fiber 102, optionalfocusing layer 110, Faraday rotator layer 112, optional spacer layer114, first waveguide 106, and second waveguide 108 are the same instructure and function as in the configuration of FIGS. 1-3.

FIG. 9 shows an example of two single-polarization grating couplers604A, 606A, suitable for use in the device 600 of FIGS. 6-8, inaccordance with some embodiments.

The first 604A of the two single-polarization grating couplers caninclude a first region in which a refractive index varies periodically.The periodic variations of the first region can be oriented orthogonalto the first redirected path 120. The first region can be sized largerthan a footprint 902 of the first and third beams at the firstsingle-polarization grating coupler 604A.

The second 606A of the two single-polarization grating couplers caninclude a second region in which a refractive index varies periodically.The periodic variations of the second region can be oriented orthogonalto the second redirected path 122. The second region can be sized largerthan a footprint 904 of the second and fourth beams at the secondsingle-polarization grating coupler 606A.

FIG. 10 shows another example of two single-polarization gratingcouplers 604B, 606B, suitable for use in the device 600 of FIGS. 6-8, inaccordance with some embodiments.

Compared with the two single-polarization grating couplers 604A, 606A ofFIG. 9, the two single-polarization grating couplers 604B, 606B of FIG.10 have the additional feature that the periodic variations inrefractive index can be curved in shape. The curvature can focus thefirst and second beams onto longitudinal ends of the first and secondwaveguides 106, 108, respectively. Specifically, in the first region,the periodic variations can be curved in shape, with a curvatureconfigured to focus the first beam onto a longitudinal end of the firstwaveguide 106. In the second region, the periodic variations can becurved in shape, with a curvature configured to focus the second beamonto a longitudinal end of the second waveguide 108. The focusingeffects of such curvature can add to or replace the focusing function ofthe optional focusing layer 110.

A potential improvement over the configuration of FIGS. 6-8 would be touse the Faraday rotator layer 112 after the birefringent layer 602,rather than before the birefringent layer 602. Doing so would rotate thepolarization of light directed back into the birefringent layer 602. Forexample, incident light in an ordinary ray would become an extraordinaryray in return light. The undesirable return light would thereforetraverse a different path within the birefringent layer 602, rather thandoubling back on itself. This allow the device to direct the returnlight to a physical location away from the fiber. Directing the returnlight away from the fiber, which can dramatically reduce the couplingefficiency back into the fiber, can provide an additional degree ofoptical isolation for the coupling device, in addition to rotating theplane of polarization.

FIG. 11 shows an example of another device 1100 that can couple incidentlight from a fiber into waveguides, but can reduce the coupling ofreturn light from the waveguides into the fiber, in accordance with someembodiments. FIG. 12 shows an example of incident light propagatingthrough the device of FIG. 11, in accordance with some embodiments. FIG.13 shows an example of return light propagating through the device ofFIG. 11, in accordance with some embodiments. FIGS. 11-13 are discussedin tandem below.

In the configuration of FIGS. 11-13, the optical fiber 102, optionalfocusing layer 110, optional spacer layer 114, first waveguide 106, andsecond waveguide 108 are the same in structure and function as in theconfiguration of FIGS. 6-8. The birefringent layer 602 and Faradayrotator layer 112 can be the same in structure, but are swapped inposition, compared to the configuration of FIGS. 6-8. In theconfiguration of FIGS. 11-13, along the first and second paths, thebirefringent layer 602 is positioned between the Faraday rotator layer112 and the optical fiber 102.

In the configuration of FIGS. 11-13, the birefringent layer 602 can bepositioned such that the Faraday rotator layer 112 is positioned betweenthe birefringent layer 602 and the redirection layer 116. Thebirefringent layer 602 can impart a spatial separation between the firstlight and the second light, such that the first light and the secondlight coincide before entering the birefringent layer 602, the firstlight is angled with respect to the second light within the birefringentlayer 602, and the first light and the second light are parallel andspatially separated between the birefringent layer 602 and the Faradayrotator layer 112.

In the configuration of FIGS. 11-13, the redirection layer 116 caninclude two single-polarization grating couplers 604, 606 having aspatial separation that matches the spatial separation between the thirdlight and the fourth light. A first 604 of the two single-polarizationgrating couplers can redirect the third light to propagate along thefirst redirected path 120. A second 606 of the two single-polarizationgrating couplers can redirect the fourth light to propagate along thesecond redirected path 122.

In a manner similar to that shown in FIG. 9, the first 604 of the twosingle-polarization grating couplers can include a first region in whicha refractive index varies periodically. The periodic variations of thefirst region can be oriented orthogonal to the first redirected path120. The second 606 of the two single-polarization grating couplers caninclude a second region in which a refractive index varies periodically.The periodic variations of the second region can be oriented orthogonalto the second redirected path 122.

In a manner similar to that shown in FIG. 10, in the first region, theperiodic variations can be curved in shape, with a curvature configuredto focus the redirected third light onto a longitudinal end of a firstwaveguide. In the second region, the periodic variations can be curvedin shape, with a curvature configured to focus the redirected fourthlight onto a longitudinal end of a second waveguide.

As best seen in FIGS. 12 and 13, the first redirected path 120 canangled at forty-five degrees with respect to a plane formed by an opticaxis (OA) of the birefringent layer 602 and a longitudinal axis (LA)that is orthogonal to a plane of the redirection layer 116. The secondredirected path 122 can be angled at forty-five degrees with respect tothe plane formed by the optic axis (OA) of the birefringent layer 116and the longitudinal axis (LA). On the first and second redirected paths120, 122, the first and second beams can have respective polarizationorientations that are parallel to the plane of the redirection layer116. Alternatively, the first and second redirected paths can be angledat ninety degrees to the orientations shown in FIGS. 12 and 13, and thefirst and second beams can have respective polarization orientationsthat are orthogonal to the plane of the redirection layer 116.

Compared with the configuration of FIGS. 6-8, the configuration of FIGS.11-13 additionally changes a physical optical path traversed by returnlight 1304 in the birefringent layer 602, which can allow the device todirect the return light 1304 to a physical location away from the fiber.Note that in FIG. 13, the fiber would be positioned directly above thegrating coupler 604, so that light from both grating couplers 604, 606is directed away from the fiber location. Directing the return light1304 away from the fiber can dramatically reduce the coupling back intothe fiber. This can provide an additional degree of optical isolationfor the coupling device 1100, in addition to rotating the plane ofpolarization.

It is instructive to consider how the devices of FIGS. 1-3, 6-8, and11-13 can be used in transmission and reception equipment.

For the configurations discussed above and shown in the figures, thethree devices can be considered to be receiving elements. Each of thedevices can receive a signal from a fiber, and can direct the receivedsignal to a suitable waveguide. For these devices, thefiber-to-waveguide path can be considered to be an incident path, andthe waveguide-to-fiber path can be considered to be a return path.

In the context of receiving elements, light propagating along the returnpath (e.g., from the waveguide to the fiber) is undesirable. The devicesabove rotate the plane of polarization of light following the returnpath, which can help reduce feedback effects. Specifically, if light isreceived with a first polarization state from the fiber, propagates fromthe fiber to the waveguide, reflects from the waveguide, and propagatesback to the fiber, the returning light has a second polarization stateorthogonal to the first polarization state. In addition, the device ofFIGS. 11-13 further imparts a physical displacement to the return light,so that light returning toward the fiber is displaced away from thefiber.

It is also possible to use these devices as transmitting elements. Fortransmission, signals originate at the waveguide, rather than at thefiber. The signals propagate along an incident path from the waveguideto the fiber, with the intent of coupling into the fiber and exiting thedevice. Reflections from the fiber (e.g., from the waveguide, to thefiber, and back to the waveguide) are undesirable.

It is possible to modify all of the devices shown above to function astransmission elements. For example, the device shown in FIG. 11 can bemodified to function as a transmission element by changing theorientations of the two grating couplers. Specifically, each gratingcoupler can be pivoted by ninety degrees about an axis orthogonal to theplane of the redirection layer. FIGS. 14 and 15 trace beams through sucha modified device.

FIG. 14 shows an example of incident light propagating through thedevice of FIG. 11 when the device of FIG. 11 is configured as atransmission element, in accordance with some embodiments.

In the geometry of the configuration of FIG. 14, a first light can bedirected into the device by a first waveguide, and can propagate towarda first grating coupler 1404 along a first optical path 1420. The firstlight can have a polarization state orthogonal to a plane of incidenceat the first grating coupler 1404, which is consistent with how thesingle-polarization grating couplers 604, 606 are used. Similarly, asecond light can be directed into the device by a second waveguide, andcan propagate toward a second grating coupler 1406 along a secondoptical path 1422. The second light can have a polarization stateorthogonal to a plane of incidence at the second grating coupler 1406,which is also consistent with how the single-polarization gratingcouplers 604, 606 are used. The first and second grating couplers 1404,1406 redirect the first and second light toward a Faraday rotator layer112. The Faraday rotator layer 112 can rotate by forty-five degrees,with a first handedness, respective planes of polarization of the firstand second lights to form respective third and fourth lights. Thebirefringent layer 602 can direct the third light as an extraordinaryray through the birefringent layer 602, and direct the fourth light asan ordinary ray through the birefringent layer 602. After passingthrough the birefringent layer 602, the third and fourth lights coincideat location 1402, and can both couple into a fiber.

FIG. 15 shows an example of return light propagating through the deviceof FIG. 11 when the device of FIG. 11 is configured as a transmissionelement, in accordance with some embodiments. The return light shown atlocation 1502 in FIG. 15 has returned from the fiber without the fibermodifying its polarization state.

Light that passes through the first grating coupler 1404 in the incidentdirection can return to the first grating coupler 1404 in the returndirection, but returns with a polarization orientation orthogonal to theincident direction. Similarly, light that passes through the secondgrating coupler 1406 in the incident direction can return to the secondgrating coupler 1406 in the return direction, but returns with apolarization orientation orthogonal to the incident direction. In bothof these cases, the light returns with a polarization orientationparallel to the plane of incidence at the first and second gratingcouplers 1404, 1406.

In general, it is difficult to design grating couplers to have highcoupling efficiency for two different polarization states, as discussedabove. The single-polarization grating couplers shown herein can bedesigned to have high coupling efficiency for just one polarizationstate, without concern for the orthogonal polarization state. Thegrating couplers 1404, 1406 can be designed to be single-polarizationgrating couplers, so that they have high coupling efficiency for lightin the incident path. For the configuration of FIG. 15, it can bedesirable to have a relatively low coupling efficiency for lightpolarization parallel to the plane of incidence at the grating couplers.Such a relatively low coupling efficiency can beneficially reduce theamount of light directed back to the waveguides along paths 1520 and1522.

The configuration of FIGS. 14 and 15 can therefore achieve isolationthrough two mechanisms: rotating the polarization state of return lightby ninety degrees, and reducing the intensity of the return light byusing grating couplers having intentionally low coupling efficienciesfor particular polarization states.

FIG. 16 shows an example of a method 1600 for coupling incident lightfrom a fiber into waveguides, and reducing the coupling of return lightfrom the waveguides into the fiber, in accordance with some embodiments.In some examples, the method of FIG. 16 can be used for coupling a firstbeam having a first polarization orientation from an optical fiber alonga first path into a first waveguide, coupling a second beam having asecond polarization orthogonal to the first polarization from theoptical fiber along a second path into a second waveguide, reducing thecoupling of a third beam having the first polarization orientation fromthe first waveguide along the first path into the optical fiber, andreducing the coupling of a fourth beam having the second polarizationorientation from the second waveguide along the second path into theoptical fiber. Such a method can be executed by any of the devices shownin FIGS. 1-13 and discussed above. The method 1600 of FIG. 16 is but oneexample of such a method; other methods can also be used.

At operation 1602, a Faraday rotator layer can receive first light andsecond light. The first light can have a first polarization orientation.The second light can have a second polarization orientation orthogonalto the first polarization orientation.

At operation 1604, the Faraday rotator layer can propagate the firstlight through the Faraday rotator layer to form third light. The thirdlight can have a third polarization orientation angled by forty-fivedegrees with respect to the first polarization orientation.

At operation 1606, the Faraday rotator layer can propagate the secondlight through the Faraday rotator layer to form fourth light. The fourthlight can have a fourth polarization orientation angled by forty-fivedegrees with respect to the second polarization orientation.

At operation 1608, a planar redirection layer including at least onegrating coupler can redirect the third light to propagate along a firstredirected path within the redirection layer and redirect the fourthlight to propagate along a second redirected path within the redirectionlayer. The second redirected path can be angled with respect to thefirst redirected path.

In some examples, such as the configuration shown in FIGS. 1-3, thefirst light and the second light can be coincident along a singleoptical path. The redirection layer can include a dual-polarizationgrating coupler. The dual-polarization grating coupler can redirect thethird light to propagate along the first redirected path within theredirection layer and redirect the fourth light to propagate along thesecond redirected path within the redirection layer.

In some examples, such as the configuration shown in FIGS. 6-8, thefirst light and the second light can be coincident along a singleoptical path. A birefringent layer can be positioned between the Faradayrotator layer and the redirection layer. The birefringent layer canimpart a spatial separation between the third light and the fourthlight, such that the third light and the fourth light coincide uponexiting the Faraday rotator layer, the third light is angled withrespect to the fourth light within the birefringent layer, and the thirdlight and the fourth light are parallel and spatially separated betweenthe birefringent layer and the redirection layer.

In some examples, such as the configuration shown in FIGS. 11-13, abirefringent layer can be positioned such that the Faraday rotator layeris positioned between the birefringent layer and the redirection layer.The birefringent layer can impart a spatial separation between the firstlight and the second light, such that the first light and the secondlight coincide before entering the birefringent layer, the first lightis angled with respect to the second light within the birefringentlayer, and the first light and the second light are parallel andspatially separated between the birefringent layer and the Faradayrotator layer.

To further illustrate the device and related method disclosed herein, anon-limiting list of examples is provided below. Each of the followingnon-limiting examples can stand on its own, or can be combined in anypermutation or combination with any one or more of the other examples.

In Example 1, a device can include a Faraday rotator layer configuredto: receive first light having a first polarization orientation, receivesecond light having a second polarization orientation orthogonal to thefirst polarization orientation, propagate the first light through theFaraday rotator layer to form third light, the third light having athird polarization orientation angled by forty-five degrees with respectto the first polarization orientation, and propagate the second lightthrough the Faraday rotator layer to form fourth light, the fourth lighthaving a fourth polarization orientation angled by forty-five degreeswith respect to the second polarization orientation; and a planarredirection layer including at least one grating coupler, theredirection layer configured to redirect the third light to propagatealong a first redirected path within the redirection layer and redirectthe fourth light to propagate along a second redirected path within theredirection layer, the second redirected path being angled with respectto the first redirected path.

In Example 2, the device of Example 1 can optionally be configured suchthat the first redirected path is orthogonal to the second redirectedpath.

In Example 3, the device of any one of Examples 1-2 can optionally beconfigured such that on the first and second redirected paths, theredirected third light and the redirected fourth light have respectivepolarization orientations that are parallel to a plane of theredirection layer.

In Example 4, the device of any one of Examples 1-3 can optionallyfurther include a focusing layer configured to: impart a convergenceonto the first light such that the redirected third light is focusedonto a longitudinal end of a first waveguide, and impart a convergenceonto the second light such that the redirected fourth light is focusedonto a longitudinal end of a second waveguide.

In Example 5, the device of any one of Examples 1-4 can optionally beconfigured such that the first light and the second light are coincidentalong a single optical path; and the redirection layer includes adual-polarization grating coupler, the dual-polarization grating couplerconfigured to redirect the third light to propagate along the firstredirected path within the redirection layer and redirect the fourthlight to propagate along the second redirected path within theredirection layer.

In Example 6, the device of any one of Examples 1-5 can optionally beconfigured such that the dual-polarization grating coupler includes aregion in which a refractive index varies periodically in twodirections, a first of the two directions being orthogonal to the firstredirected path, a second of the two directions being orthogonal to thesecond redirected path.

In Example 7, the device of any one of Examples 1-6 can optionally beconfigured such that the periodic variations in refractive index arecurved in shape, with a curvature configured to: focus the redirectedthird light onto a longitudinal end of a first waveguide, and focus theredirected fourth light onto a longitudinal end of a second waveguide.

In Example 8, the device of any one of Examples 1-7 can optionally beconfigured such that the first light and the second light are coincidentalong a single optical path; and further including a birefringent layerpositioned between the Faraday rotator layer and the redirection layer,the birefringent layer configured to impart a spatial separation betweenthe third light and the fourth light, such that: the third light and thefourth light coincide upon exiting the Faraday rotator layer, the thirdlight is angled with respect to the fourth light within the birefringentlayer, and the third light and the fourth light are parallel andspatially separated between the birefringent layer and the redirectionlayer.

In Example 9, the device of any one of Examples 1-8 can optionally beconfigured such that the redirection layer includes twosingle-polarization grating couplers having a spatial separation thatmatches the spatial separation between the third light and the fourthlight, a first of the two single-polarization grating couplersconfigured to redirect the third light to propagate along the firstredirected path, a second of the two single-polarization gratingcouplers configured to redirect the fourth light to propagate along thesecond redirected path, the first of the two single-polarization gratingcouplers including a first region in which a refractive index variesperiodically, the periodic variations of the first region being orientedorthogonal to the first redirected path, the second of the twosingle-polarization grating couplers including a second region in whicha refractive index varies periodically, the periodic variations of thesecond region being oriented orthogonal to the second redirected path.

In Example 10, the device of any one of Examples 1-9 can optionally beconfigured such that in the first region, the periodic variations arecurved in shape, with a curvature configured to focus the redirectedthird light onto a longitudinal end of a first waveguide, and in thesecond region, the periodic variations are curved in shape, with acurvature configured to focus the redirected fourth light onto alongitudinal end of a second waveguide.

In Example 11, the device of any one of Examples 1-10 can optionally beconfigured such that the first redirected path lies within a planeformed by an optic axis of the birefringent layer and a longitudinalaxis that is orthogonal to a plane of the redirection layer; the secondredirected path is orthogonal to the plane formed by the optic axis andthe longitudinal axis; and on the first and second redirected paths, theredirected third light and the redirected fourth light have respectivepolarization orientations that are parallel to a plane of theredirection layer.

In Example 12, the device of any one of Examples 1-11 can optionallyfurther include a birefringent layer positioned such that the Faradayrotator layer is positioned between the birefringent layer and theredirection layer, the birefringent layer configured to impart a spatialseparation between the first light and the second light, such that: thefirst light and the second light coincide before entering thebirefringent layer, the first light is angled with respect to the secondlight within the birefringent layer, and the first light and the secondlight are parallel and spatially separated between the birefringentlayer and the Faraday rotator layer, wherein: the redirection layerincludes two single-polarization grating couplers having a spatialseparation that matches the spatial separation between the third lightand the fourth light; a first of the two single-polarization gratingcouplers is configured to redirect the third light to propagate alongthe first redirected path; a second of the two single-polarizationgrating couplers is configured to redirect the fourth light to propagatealong the second redirected path; the first of the twosingle-polarization grating couplers includes a first region in which arefractive index varies periodically, the periodic variations of thefirst region are oriented orthogonal to the first redirected path; thesecond of the two single-polarization grating couplers includes a secondregion in which a refractive index varies periodically, the periodicvariations of the second region are oriented orthogonal to the secondredirected path; the first redirected path is angled at forty-fivedegrees with respect to a plane formed by an optic axis of thebirefringent layer and a longitudinal axis that is orthogonal to a planeof the redirection layer; the second redirected path is angled atforty-five degrees with respect to the plane formed by the optic axis ofthe birefringent layer and the longitudinal axis; and on the first andsecond redirected paths, the first and second beams have respectivepolarization orientations that are parallel to the plane of theredirection layer.

In Example 13, the device of any one of Examples 1-12 can optionally beconfigured such that in the first region, the periodic variations arecurved in shape, with a curvature configured to focus the redirectedthird light onto a longitudinal end of a first waveguide, and in thesecond region, the periodic variations are curved in shape, with acurvature configured to focus the redirected fourth light onto alongitudinal end of a second waveguide.

In Example 14, a method can include receiving first light and secondlight at a Faraday rotator layer, the first light having a firstpolarization orientation, the second light having a second polarizationorientation orthogonal to the first polarization orientation;propagating the first light through the Faraday rotator layer to formthird light, the third light having a third polarization orientationangled by forty-five degrees with respect to the first polarizationorientation; propagating the second light through the Faraday rotatorlayer to form fourth light, the fourth light having a fourthpolarization orientation angled by forty-five degrees with respect tothe second polarization orientation; and with a planar redirection layerincluding at least one grating coupler, redirecting the third light topropagate along a first redirected path within the redirection layer andredirecting the fourth light to propagate along a second redirected pathwithin the redirection layer, the second redirected path being angledwith respect to the first redirected path.

In Example 15, the method of Example 14 can optionally be configuredsuch that the first light and the second light are coincident along asingle optical path; wherein the redirection layer includes adual-polarization grating coupler and further including with thedual-polarization grating coupler, redirecting the third light topropagate along the first redirected path within the redirection layerand redirecting the fourth light to propagate along the secondredirected path within the redirection layer.

In Example 16, the method of any one of Examples 14-15 can optionally beconfigured such that the first light and the second light are coincidentalong a single optical path; and further comprising: with a birefringentlayer positioned between the Faraday rotator layer and the redirectionlayer, imparting a spatial separation between the third light and thefourth light, such that: the third light and the fourth light coincideupon exiting the Faraday rotator layer, the third light is angled withrespect to the fourth light within the birefringent layer, and the thirdlight and the fourth light are parallel and spatially separated betweenthe birefringent layer and the redirection layer.

In Example 17, the method of any one of Examples 14-16 can optionallyfurther include with a birefringent layer positioned such that theFaraday rotator layer is positioned between the birefringent layer andthe redirection layer, imparting a spatial separation between the firstlight and the second light, such that: the first light and the secondlight coincide before entering the birefringent layer, the first lightis angled with respect to the second light within the birefringentlayer, and the first light and the second light are parallel andspatially separated between the birefringent layer and the Faradayrotator layer.

In Example 18, a layered optical device can include a birefringent layerconfigured to receive light along a single optical path and spatiallyseparate the received light into a first beam and a second beam, thefirst and second beams having polarization states that are orthogonal toeach other; a Faraday rotator layer configured to: rotate a plane ofpolarization of the first beam by forty-five degrees to form a thirdbeam, and rotate a plane of polarization of the second beam byforty-five degrees to form a fourth beam, the third and fourth beamshaving polarization states that are orthogonal to each other; and aplanar redirection layer including: a first grating coupler configuredto redirect the third beam to propagate along a first redirected pathwithin the redirection layer; and a second grating coupler configured toredirect the fourth beam to propagate along a second redirected pathwithin the redirection layer, the first and second redirected pathsbeing orthogonal to each other.

In Example 19, the layered optical device of Example 18 can optionallybe configured such that the first grating coupler is configured suchthat the third beam has a polarization state that is orthogonal to aplane formed by the third beam and the first redirected path; and thesecond grating coupler is configured such that the fourth beam has apolarization state that is orthogonal to a plane formed by the fourthbeam and the second redirected path.

In Example 20, the layered optical device of any one of Examples 18-19can optionally be configured such that the first grating coupler isconfigured such that the third beam has a polarization state that isparallel to a plane formed by the third beam and the first redirectedpath; and the second grating coupler is configured such that the fourthbeam has a polarization state that is parallel to a plane formed by thefourth beam and the second redirected path.

What is claimed is:
 1. A device comprising: an optical rotation layerconfigured to receive a light including a first light and a second lighthaving different initial polarizations, the optical rotation layerconfigured to rotate the light such that the first light and the secondlight are rotated to different polarizations; a birefringent layerconfigured to separate the rotated light into a rotated first light anda rotated second light that are separated by a physical distance; and aredirection layer configured to redirect the rotated and separated firstlight to a first redirected path and further redirect the rotated andseparated second light to a second redirected path.
 2. The device ofclaim 1, wherein the redirection layer comprises a first coupler and asecond coupler separated in the redirection layer by the physicaldistance.
 3. The device of claim 2, wherein the first coupler is a firstgrating and the second coupler is a second grating.
 4. The device ofclaim 3, wherein the first grating directs the rotated and separatedfirst light towards the first redirected path and wherein the secondgrating directs the rotated and separated second light towards thesecond redirected path.
 5. The device of claim 1, further comprising afocusing layer with a lens that directs the first light and the secondlight through the optical rotation layer, through the birefringencelayer, towards the redirection layer.
 6. The device of claim 1, whereinthe first light and the second light are input to the device by an inputwaveguide that co-propagates the first light and the second light. 7.The device of claim 6, wherein the input waveguide is a fiber.
 8. Thedevice of claim 1, wherein the first light has a first initialpolarization and the second light has a second initial polarization, thefirst initial polarization being different than the second initialpolarization.
 9. The device of claim 8, wherein the optical rotatorlayer is configured to rotate the first light from the first initialpolarization to a first rotated polarization and rotate the second lightfrom the second initial polarization to a second rotated polarization,the first rotated polarization being different than the second rotatedpolarization.
 10. The device of claim 1, wherein the optical rotatorlayer is a Faraday optical rotator.
 11. The device of claim 10, whereinthe Faraday optical rotator is configured to rotate polarizations of thefirst light and the second light by 45 degrees.
 12. The device of claim1, wherein the birefringent layer comprises negative uniaxialbirefringent material to separate the first light and the second light.13. The device of claim 1, wherein the birefringent layer comprisespositive uniaxial birefringent material to separate the first light andthe second light.
 14. A method for routing light using an opticalcoupler device comprising: receiving, by an optical rotation layer ofthe optical coupler device, a light including a first light and a secondlight having different initial polarizations; rotating, by the opticalrotation layer, the light such that the first light and the second lightare rotated to different polarizations; separating, by a birefringentlayer of the optical coupler device, the rotated light into a rotatedfirst light and a rotated second light that are separated by a physicaldistance; and redirecting, by a redirection layer of the optical couplerdevice, the rotated and separated first light to a first redirected pathand the rotated and separated second light to a second redirected path.15. The method of claim 14, wherein the redirection layer comprises afirst grating and a second grating separated in the redirection layer bythe physical distance, wherein the first grating directs the rotated andseparated first light towards the first redirected path and wherein thesecond grating directs the rotated and separated second light towardsthe second redirected path.
 16. The method of claim 14, furthercomprising: directing, by a focusing layer of the optical couplerdevice, the first light and the second light through the opticalrotation layer, through the birefringence layer, towards the redirectionlayer.
 17. The method of claim 14, wherein the first light has a firstinitial polarization and the second light has a second initialpolarization, the first initial polarization being different than thesecond initial polarization, and wherein the optical rotator layer isconfigured to rotate the first light from the first initial polarizationto a first rotated polarization and rotate the second light from thesecond initial polarization to a second rotated polarization, the firstrotated polarization being different than the second rotatedpolarization.
 18. The method of claim 14, wherein the optical rotatorlayer is configured to rotate polarizations of the first light and thesecond light by 45 degrees.
 19. The method of claim 14, wherein thefirst light and the second light are received by an input waveguide ofthe optical coupler device that co-propagates the first light and thesecond light.
 20. The method of claim 19, wherein the input waveguide isa fiber.